or Saussurite-smaragdite Gabbro of the Saasthal. 247 



Yet this region, as a whole, undoubtedly has been subjected 

 to very severe pressure, of which frequently there are con- 

 spicuous indications — for in some places serpentine has been 

 crushed into a slaty schist, other rather less basic rocks into 

 chloritic or hornblendic schists, granitoid rocks into slabby 

 gneisses with " sheen surfaces,'"' &c* 



Still, among these euphotides, certain specimens of those 

 containing green hornblende (No. 4) present structures which 

 make it possible that the mass was somewhat modified by 

 crushing. This is the case with the small specimen from 

 the spur of the Allaleinhorn, given to me by Mr. Eccles ; also, 

 though to a less extent, in sundry specimens collected by 

 myself, one from the ridge, another from one of the bosses. 

 But in the majority of the boulders of this type I could find 

 no direct evidence in favour of their slight " flaser " structure 

 being due to this cause, while occasionally their structures 

 seemed readily explicable by fiuxional movements, as is the 

 case with certain of the gabbros at the Lizard. Hence, while 

 I should admit that this euphotide had been occasionally 

 modified by subsequent pressure, I should infer that on the 

 whole it has done little more than modify structures which 

 were original, and that these have been generally unaffected 

 by the process of mountain-making. 



From these observations it follows indirectly that gabbro is 

 by no means a promising subject for " dynamo -metamor- 

 phisrn/' and to attribute a flaser structure to this agency, 

 when no mark has been left on neighbouring and less obstinate 

 rocks, is to quit induction for hypothesis! , 



Two other matters, in connexion with these boulders, appear 

 to me to call for a few remarks in conclusion : — 



(a) In all parts of the Saasthal which I have examined, 

 the boulders containing smaragdite are more abundant than 

 those in which the dull green hornblende dominates (4) . But, 

 as I have stated, this is not the case on the ridge itself. The 

 smaragdite, no doubt, may occur in parts which I did not see, 

 as it does on the crags below the eastern end, but on the crest 



* The u cleavage foliation " is often so marked that the slabs are used 

 for flooring and occasionally even for roofing. 



t I have seen statements in print so worded as to suggest that a 

 u uralitization " or conversion of augite into hornblende is indicative of 

 (t dynaino-metamorphism." It is true that, as augite is a less stable 

 mineral than hornblende, any disturbing agency is likely to set up the 

 change from one condition to another, and so very often it results from 

 dynamo-metamorphism, but I have found instances of this change when 

 there was not the slightest indication of mechanical disturbance to be 

 seen in the neighbourhood. Geologists would do well to remember that 

 though Acauses B, B is not always caused by A. 



